The First Meiotic Division

The first meiotic division is subdivided into several stages - prophase, metaphase, anaphase and telophase - usually denoted by 1 (e.g. prophase 1) as they are part of the first meiotic division.

Prophase is further subdivided into leptotene (early prophase), zygotene (middle prophase), pachytene, diplotene (late prophase) and diakinesis. It is a more complicated and slower process than the equivalent stage in mitosis and the prophase of meiosis 2, and so takes up the greatest part of the 18.7 days for the first meiotic division. Preleptotene primary spermatocytes immediately begin meiosis by active DNA synthesis. This may take 14-28 h (depending upon the species) and accounts for the relative slowness of prophase 1 (Courot et al., 1970). Chromatin clumps may be found near the nuclear membrane of these preleptotene primary spermatocytes and subsequently disperse to produce the fine chromatin filaments evident in leptotene primary spermatocytes. These filaments may not be evenly distributed and so the nucleus of leptotene primary spermatocytes may appear asymmetrical (Johnson, 1991b). During this leptotene stage the chromosomes elongate and become visible as apparently single structures. However, by this stage, most (if not all) of the DNA has been doubled during the preleptotene phase, evident from the doubling of the total DNA content. These structures are termed univalents (Kimball, 1983).

During zygotene the chromosomes within the cell pair, lengthwise with their homologue, in a process termed synapsis. This pairing of chromosomes allows for the exchange of genetic material between different or non-sister

Meiosis Stages Their
Fig. 4.20. The process of meiosis that results in spermatogonia in the stallion.

chromosomes (that is, of maternal and paternal origin) at attachment sites termed synaptonemal complexes. At this stage the two sex chromosomes, XX or XY, are isolated into a sex vesicle (Burgos et al., 1970).

The pachytene phase follows, commencing with large chromosomes. During this phase each chromosome becomes evident as two chromatids and shortens in length. Pachytene is the longest phase and the exchange of genetic material started in zygotene continues.

The diplotene stage is marked by the pulling apart of the two homologues. At this stage the fact that each homologue is made up of two chromosomes, which are in turn made up of two sister chromatids, becomes evident. The pulling apart is not complete and two areas of attachment remain. Firstly, the attachment of sister chromatids at their centriole persists and, secondly, the attachment of the non-sister chromatids is evident at points termed chiasma. These points mark the position where cross-over of segments of non-sister chromatids has occurred. This cross-over is reciprocal, in that the total genetic material in each non-sister chromatid remains unaltered after the process is complete. There are normally several chiasma in a single bivalent and, as can be imagined, there are numerous permutations from the cross-overs between the four chromatids. The only restrictions placed on cross-overs are that they do not occur between sister chromatids and there is not a simultaneous crossover of more than two chromatids at a single point at a set time (Kimball, 1983; Johnson, 1991b).

Diakenesis, the final stage of prophase 1, is relatively short lived. During diakenesis the homologous partners continue to separate and the chromatids continue to shorten.

Metaphase 1 marks the disappearance of the nuclear membrane and the appearance of a spindle on which the chromosomes arrange themselves. Each pair of homologues arrange themselves on either side of the equator of the spindle, with their centromeres acting as attachment points. This random arrangement of homologues allows for further genetic variability in the resultant cells. As it is exceedingly rare for all univalents of paternal origin to locate themselves on one side and univalents of maternal origin on the other, a random arrangement occurs (Kimball, 1983; Phillips and Chilton, 1991).

Anaphase 1 heralds the migration of the centromere of each bivalent to either pole (end) of the cell, thus separating the bivalents into univalents again. The centromeres continue to attach each pair of sister chromatids together (Kimball, 1983; Phillips and Chilton, 1991).

Telophase 1 is the continuation of the separation of the bivalents started in anaphase 1, resulting in two separated cells, each having only one member of each of the original homologous pair. The division has not been just the straightforward division of a single bivalent (maternal and paternal) into two univalents, as there has been an exchange of genetic material and chromatid segments along the way. Telophase 1 results in the production of two haploid secondary spermatocytes. These spermatocytes then enter the second meiotic division (Kimball, 1983).

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